This invention relates to a suspension control device in a vehicle, and more particularly to a suspension control device which is effective in suppressing shocks applied intermittently to a vehicle traveling on a road by the unevenness of the road, and the vibration of the vehicle following a shock.
A suspension control device has been disclosed, for instance, by Japanese Patent Application (OPI) No. 166715/1986 (the term "OPI" as used herein means an "unexamined published application"). The conventional suspension control device, as shown in FIG. 8, comprises: front wheel acceleration detecting means M2 for detecting an acceleration in a direction perpendicular to the road surface which is applied to the front wheels M1 of the vehicle; decision means M3 for deciding whether or not an acceleration detected by the front wheel acceleration detecting means M2 is out of a predetermined range; and rear wheel suspension characteristic changing means M5 for changing the suspension characteristic of the rear wheels M4 when the decision means M3 has decided that the acceleration is out of the predetermined range.
In the suspension control device thus designed, the front wheel acceleration detecting means M2 detects an acceleration in a direction perpendicular to the road surface, and the decision means M3 decides a degree of unevenness of the road surface corresponding to the acceleration thus detected. The result of decision is applied to the rear wheel suspension characteristic changing means M5. When the degree of unevenness of the road surface is larger than a predetermined value, the rear wheel suspension characteristic changing means M5 operates to suppress the vibration of the rear wheels M4, or to ensure the controllability and stability for the vehicle body.
The above-described conventional suspension control device employs a system such that the changing of the suspension characteristics are performed based on a moving of the front wheels and the vertical acceleration, and the passing timing of the rear wheels over the obstruction is determined from the detection of the vehicle speed, to restore the suspension characteristics. Therefore, the conventional suspension control device suffers from a problem that the changing of the characteristics may be performed independently of the vibration of the vehicle body, this results in an unreliability of the control. In the case where obstructions appear periodically such as the seams of the road surfaces of a highway, the suspensions must be controlled frequently in synchronization with the appearance of the seams of the road surfaces irrespective of the vibration of the vehicle body. If the seams of the road surfaces are not positively detected, then it is difficult to effectively suppress the resonance of the vehicle body and to lessen the shocks given to it by the road surface.
Furthermore, in the case where obstructions appear frequently, even when the vehicle body suffers from resonance, there is no time to control the damping force and to make the suspension setting hard. In addition, it cannot be determined without fail only from signals stemming from obstructions that harmful vibrations will occur.